It goes something like this: if you retrieve food dropped on the floor or another surface within three or five seconds, it won't yet be contaminated with bacteria.
Okay, it might just be wishful thinking - or a handy excuse not to throw otherwise good food in the bin. But it really doesn't add up.
Let's look at the facts. Disease-causing bacteria (known as pathogenic bacteria) and other microorganisms (such as viruses) are potentially everywhere and can be remarkably virulent. So logically, it would be an easy task for a microorganism to attach itself to a surface, especially to a moist piece of food.
A 2007 study Journal of Applied Microbiology paper from a team at Clemson University in the United States, tested the five-second rule on tile, wood and carpet. They contaminated the three surfaces with a high level of Salmonella Typhimurium and looked at the rate in which the bacteria transferred to bread and sausages, over a period of 24 hours.
They found the most significant variable in the transfer rates from all three surfaces was not the length of time it had contact with the food. The three testing times (five, 30 or 60 seconds) made little difference in the rate of bacterial transfer.
The length of time the bacteria had been on the surface prior to contact with the food mattered more. Four hours after contamination, the same amount of bacteria remained on the carpet, while rates of bacteria on the tile and wood were slightly lower.
But another study, on bacteria in the manufacturing environment, found that the longer the food was exposed to a contaminated surface, the more bacteria it accumulated - as did an investigation on transfer between meat surfaces.
Overall, a comprehensive review on bacterial attachment to surfaces concluded that moisture, pressure and contact time increased the likelihood of bacterial transfer.
Watch what you touch
Countless studies have reported that pathogenic bacteria and viruses have a long life on inanimate objects, such as paper and public telephones and in various public places such as classrooms, homes, offices, shops, playgrounds and other environments.
This is of particular concern because if transfer rates to food are similarly high, then there's potential for contamination of food from bacteria and other microorganisms present on inanimate objects.
Let's look at a specific personal inanimate object, regarded as indispensable in modern society, handled frequently during the day, held close to the face and placed on many surfaces – the mobile phone.
Many may not consider their mobile phone as a source of microbial contamination and disease potential, but studies have shown otherwise.
A 2011 study of Ghana university students found all 100 mobile phones inspected were contaminated with bacteria and many contained recognisable pathogens. One-quarter of the mobiles had Bacillus cereus (responsible for food poisoning) and one-fifth had Proteus mirabilis (which can cause urinary tract infections).
Similarly, the London School of Tropical Medicine and Hygiene recently reported that one in six mobile phones in the United Kingdom is contaminated with faecal matter, including pathogenic E. coli.
Consequently, while some people hold onto the belief that food is safe to eat after falling on the floor, the take-home message here relates more to mobile phone surfaces than floor surfaces.
Don't eat food that has fallen on your mobile phone (unless you've just cleaned it!) and don't eat food with your fingers if you've held your mobile phone.
My advice? Give your mobile phone a wipe down with a moistened cloth containing an antibacterial chemical when you get home each day. That way, you can minimise the chances that pathogenic bacteria, viruses or other microorganisms from your day's outing aren't transferred to your home's inanimate surfaces.
And for those who hold on to the three-second rule, there'll be fewer germs to contaminate your food.